“Dry Limestone Grinding Makes Sense” - Hot Topic Hour Thursday, January 26

This was the message delivered by Greg Anderson, the speaker from Mississippi Lime in the Hot Topic Hour yesterday. He said that dry grinding with roller mills makes sense if you are going to grind at the plant site. It is especially attractive to consider not grinding at all but to pay for delivered ground limestone. Regional dry grinding can save up to $50 million in up front capital cost. The additional payments to the lime producer are more than offset by electricity consumption, depreciation and other operating costs of a wet ball mill system.

Bob McIlvaine asked Greg to discuss the various ramifications if a plant decided to switch to lime. The dedicated dry system would be more amenable to a switch. Obviously if the product is purchased then the switch would be even easier.

The purpose of the question is the potential need for higher efficiency. There was no extensive discussion of this in the webinar but it is an important consideration. Because of PM _2.5 and other ambient air quality rules, it seems inevitable that states will force utilities to achieve the very highest SO₂ removal efficiencies. A one percent efficiency difference for a 1,000 MW high sulfur coal unit would offset all the barbecues and most of the industrial boiler emissions in a state.

Let’s also take a step back and say, what is the long term goal for coal?  Many would like to see coal disappear completely. No one can defend the logic of allowing old coal-fired power plants to emit more than new coal-fired power plants, so it is inevitable that eventually every plant will be pushed to maximize SO₂ removal efficiency.

Lime based systems can achieve 99 percent SO₂ removal. Some German companies such as Evonik have multiple reagent capable systems. They can use limestone, lime, or marl. The advantage is flexibility as reagent prices change but also fuel flexibility. They can switch to a higher sulfur coal and lime and still meet the emission limits.

There is a new dimension. The newer design limestone systems can also now meet the 99 percent removal rates. Both Alstom and Hitachi presented papers at Power-Gen showing that this high rate is being achieved thanks to improvement in the spray tower system design. Coverage of these two papers will appear next week in our FGD & DeNOx Newsletter.

The changes to the limestone system to reach higher efficiency are not substantial. Likewise the lime option can also be cost effective. You can use lime and much smaller scrubbers and lower capital cost to meet 99 percent removal, so it will be in the best interest of the utilities, the coal industry, and the suppliers to pursue these high efficiency levels.